Devices for the recording of digital image data to an image-receiving medium are known, such as the documents WO 02/13511, U.S. Pat. No. 5,315,322, U.S. Pat. No. 6,175,666, JP 2OO1213007, 2OO1228619, JP 2OO1245485 or JP 61019278.
The apparatus of the preamble typically comprises a memory, which is able to store and retrieve the complete image data without gaps and at the required data rate, and a printing head, which records fed image data via a controlled motion device along two directions in defined and precise order onto the image-receiving medium. The image-receiving medium can be, for example, a photosensitive material, paper, special foil, printing plate or the like. To ensure that the physical image on the image material is generated completely with all applied pixels in its original quality, the movement of the recording head along the two axes has to be detected via position sensors to be used by means of suitable processing to synchronize the data retrieval.
In common methods and devices for recording of digital data on an image-receiving medium the image recording occurs by means of a fast movement of a recording head along an image line, the so-called fast-scan axis, and by means of a slow movement of the image-receiving medium along the so-called slow-scan axis, which is orthogonal to the image line. The fast movement is typically provided by the rotation of an opto-mechanical assembly. The slow movement is typically provided by a continuous or step-wise movement of the image-receiving medium relative to the recording head or vice-versa. The size of the pixels as well as their positioning accuracy during recording of the digital data decisively determine the quality and resolution of the generated complete image. To obtain a resolution of e.g. 8000 dpi or 160 line pairs per millimeter, the recording head has to provide pixels with a diameter of 3 μm and write them with a position tolerance of typically 300 nm onto a perfect two-dimensional grid. For example, to achieve a resolution of 8000 dpi in a laser recorder when recording digital data on an image-receiving medium, the laser beam has to be focused exactly into the photosensitive layer to a full width at 1/e2 diameter of approximately 6 μm (corresponding to a full half value width of 3 μm). To pixels of an uniform size of 3 μm, the waist of the Laser beam has to be kept inside the photosensitive layer over the whole image area with tight tolerances; according to Rayleigh, in order to maintain a tolerance of the diameter of the pixels of 2%, the waist of the Gaussian laser beam must be maintained with approximately 10 μm accuracy within the photosensitive layer. Therefore, for a high resolution illumination, the image-receiving medium Is preferably sucked onto a support surface, e.g. onto the interior of a drum.
Further it must be observed that the movement of the recording head relative (or printing head) relative to the image-receiving medium will lead to a spreading of the applied light energy (the applied pixel) in case the exposure period for one pixel is not substantially shorter than the time it takes for the focused laser beam to move from one pixel position to the next along on the desired printing matrix along the fast-scan direction.
At a typical velocity of the Laser beam of 100 m/s along the fast-scan-direction a pixel distance of 3 μm is covered in 30 ns. In laser recording devices acousto-optical modulators are usually used, which deliver, at best, have rise and fall times of about 5 ns, which allows to reach shortest illumination intervals of approximately 15 ns. This illustration shows that the pixels on the photosensitive image-receiving medium are illuminated in circular manner only if the recording Laser beam in the illumination point has a suitable shape.
In a high resolution recording procedure with e.g. 8,000 dpi about 100,000 pixels are to be recorded of the length of a line of 300 mm, and in particular even the last pixel in each line must not have more than 300 nm positional deviation. Consequently, a relative positional tolerance of 10−6 (1 ppm, part per million) has to be maintained along the fast-scan direction. For known recording systems with a rotating recording head the speed of rotation can be kept constant with a tolerance of typically 5*10−5 (50 ppm) by means of a precise control of the drive. While the tolerance requirements for the velocity of the recording head along the slow-scan direction can be fulfilled by means according to prior art, there is a need for novel methods and devices for moving along the fast-scan direction.